The first mishnah of this tractate introduced the concept of an unbroken chain of personalities through whom and to whom the oral tradition which had originated at Sinai was passed on. The human links in this chain were traced from Moses through the prophetic era down to the members of the Great Assembly which functioned in the first centuries after the return of the exiles from Babylon to Judah. The next link in the chain was Simon the Righteous, who presided over the historic developments which saw the Great Assembly evolve into a Sanhedrin. He was followed by Antigonos of Sokhoh and, as our present mishnah teaches, he had two successors, not just one. Indeed, we now enter a period of nearly two hundred years during which the leadership of the Pharisaic movement was shared between two leaders in each generation. This era of dual leadership we traditionally call the Age of the Pairs [Zugot].
2:
It is a novel situation in which a society is ruled by two leaders simultaneously. I do not think that the idea came to the leaders of the Pharisees from the consulate of the ancient Roman Republic (though the Hasmonean family during the period under discussion had established diplomatic relations of a certain kind with Rome). The dual leadership of the "Pairs" came about as an organizational or sociological necessity. We must recall that the Pharisaic movement was a hybrid movement and not uniform in its membership. First of all there were the sages who upheld the validity and essentiality of the oral tradition, Torah she-b'al-peh. Then, at a moment of crisis, their ranks had been strengthened by the defection of Simon the Righteous and his colleagues from the Sadducean camp [see Avot008 for the details]. But there was now not only a dichotomy between the more conservative views of the erstwhile Sadducees within the joint movement and the more liberal views of the sages who were members of ancient standing; there was also a difference between the wealthier members and the poorer members. The wealthier members came from two major sources. Some were of the priestly aristocracy: when they crossed the lines into the camp of the Pharisees, of course they did not forego their lineage or their rural estates. The other source of wealthy members came, of course, from the new bourgeoise, particularly in Jerusalem which was becoming a minor metropolis of international standing. On the other side of the camp were the rural peasants and the urban proletariat.
3:
Rabbi Louis Finkelstein z"l prefers to call these two groups within the Pharisaic movement Patricians and Plebeans. While the terms of this dichotomy are obviously borrowed from the sociology of the Roman Republic it is convenient to use the same terminology for our purposes, even though the terms do not carry the same emotional overtones and undertones. In order to keep the Pharisaic movement one movement it was necessary to have a dual leadership, one leader in each generation being styled the President of the Sanhedrin and the other being styled Av Bet Din - President of the Court. In all probability the former presided when the Sanhedrin was sitting as a deliberative body and the latter when it was sitting in a judicial capacity. This dual leadership is recognized in our sources, of course. The Mishnah [Ĥagigah 2:2] gives a list of the names of the five "Pairs" in chronological order, and remarks that the first name in each pair was the President and the second name was the Av Bet Din.
To be continued.